1. Field of the Invention
The present invention relates to a plasmon antenna for generating near-field light by being irradiated with light.
And the present invention relates to a head used for thermal-assisted magnetic recording in which a magnetic recording medium is irradiated with near-field light, thereby anisotropic magnetic field of the medium is lowered, thus data can be written. Further, the present invention relates to a magnetic recording apparatus provided with the head.
2. Description of the Related Art
As the recording density of a magnetic disk apparatus becomes higher, further improvement has been required in the performance of a thin-film magnetic head and a magnetic recording medium. As the thin-film magnetic head, a composite-type thin-film magnetic head is widely used, which has a stacked structure of a magnetoresistive (MR) element for reading data and an electromagnetic transducer for writing data.
Whereas, the magnetic recording medium is generally a kind of discontinuous body of magnetic microparticles gathered together, and each of the magnetic microparticles has a single magnetic domain structure. Here, one record bit consists of a plurality of the magnetic microparticles. Therefore, in order to improve the recording density, it is necessary to decrease the size of the magnetic microparticles and reduce irregularity in the boundary of the record bit. However, the decrease in size of the magnetic microparticles raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume.
As a measure against the thermal stability problem, it may be possible to increase the magnetic anisotropy energy KU of the magnetic microparticles. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. Whereas, write field intensity of the thin-film magnetic head is limited by the amount of saturation magnetic flux density of the soft-magnetic material of which the magnetic core of the head is formed. Therefore, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field (coercive force) of the medium exceeds the write field limit. Recently, as a method for solving this problem of thermal stability, so-called a thermal-assisted magnetic recording technique is proposed, in which writing is performed by reducing the anisotropic magnetic field with heat supplied to the magnetic recording medium formed of a magnetic material with a large KU just before applying write field.
As a thermal-assisted magnetic recording technique, a method has been generally known, in which a near-field light probe formed of a metal piece, so-called a plasmon antenna, is used for generating near-field light from plasmon that is excited by irradiated laser light. For example, U.S. Pat. No. 6,768,556 B1 discloses a plasmon antenna that includes a metal scatterer with a strobilus shape formed on a substrate and a dielectric material film formed around the metal scatterer. And US Patent Publication No. 2004/081031 A1 discloses a configuration in which a plasmon antenna is formed in contact with the main magnetic pole of a magnetic head for perpendicular magnetic recording in such a way that the irradiated surface of the plasmon antenna is perpendicular to the surface of a magnetic recording medium. Further, US Patent Publication No. 2003/066944 A1 discloses a technique in which the tip of a plasmon antenna is made closer to a magnetic recording medium to attempt to irradiate the medium with stronger near-field light.
However, when such a plasmon antenna is used as a near-field light generating part to implement thermal-assisted magnetic recording, a difficult problem can arise as described below.
While a plasmon antenna converts applied laser light to near-field light as described above, it is known that the light use efficiency is approximately 10% at the highest. Most part of the applied laser light, excluding the light reflected by the surface of the plasmon antenna, changes to thermal energy in the plasmon antenna. The size of the plasmon antenna is set to a value less than or equal to the wavelength of laser light, and its volume is very small. Accordingly, the thermal energy heats the plasmon antenna to an extremely high temperature. For example, a simulation shows that, when a plasmon antenna made of Au that is a 50-nm-thick equilateral-triangular plate with each edge of 300 nm (nanometers) absorbs laser light of 17 mW at room temperature, the temperature of the plasmon antenna reaches 500° C. (degrees Celsius).
Such temperature rise causes the plasmon antenna to thermally expand and protrude from the opposed-to-medium surface toward a magnetic recording medium. As a result, the end, which reaches the opposed-to-medium surface, of a read head element for reading data signal or servo signal from the magnetic recording medium can become relatively far apart from the magnetic recording medium. If this is the case, it will be difficult to properly read the servo signal during writing in which the plasmon antenna is used to irradiate the magnetic recording medium with near-field light. In addition, the electrical resistance of the plasmon antenna increases to a considerably high value at such extremely high temperature. This means that the light use efficiency of the plasmon antenna described above can be further degraded because of increased thermal disturbance of free electrons in the plasmon antenna.
To solve the problem, the present inventors have devised a near-field light generating element in which laser light propagating through a waveguide is coupled with a plasmon antenna in a surface plasmon mode to cause the excited surface plasmon to propagate to the opposed-to-medium-surface, thereby providing near-field light, rather than directly applying the laser light to a plasmon antenna. The plasmon antenna in the element will be hereinafter referred to as a surface plasmon antenna. In the near-field light generating element, the temperature of the surface plasmon antenna does not excessively rise because laser light is not directly applied to the surface plasmon antenna. Further, the portion in which laser light is coupled with the surface plasmon antenna in the surface plasmon mode is provided on the side opposite to a magnetic pole for generating write field for writing data to prevent laser light from being absorbed into the magnetic pole, thereby ensuring a certain amount of light to be applied to the surface plasmon antenna.
To perform thermal-assisted magnetic recording in practice by using the above-described near-field light generating element, the end of the surface plasmon antenna needs to be located as close to the magnetic pole end as possible in the opposed-to-medium surface. In particular, the distance between them in the direction along track is preferably set to 100 nm or less. Such a distance can provide a sufficiently large field gradient of write field generated from the magnetic pole in a position on the magnetic recording medium where near-field light is applied.
For the same reason, the emitting position on the end surface of the surface plasmon antenna where near-field light is emitted needs to be located as close to the magnetic pole as possible. To meet the requirement, it may be contemplated to make a portion of the surface plasmon antenna that is near the opposed-to-medium surface ultrathin. To make the portion near the opposed-to-medium surface ultrathin, however, an extremely high polishing accuracy must be achieved in the polishing process for forming the opposed-to-medium surface during manufacturing of the head. Therefore, this approach is difficult to implement. On the other hand, it may be also contemplated that, instead of increasing the accuracy of polishing, the angle of inclination of a propagation surface of the surface plasmon antenna in which surface plasmon propagates is reduced so that the portion near the opposed-to-medium surface is made ultrathin after polishing. However, in this case, the distance by which surface plasmon propagates increases and so does the amount of energy absorbed into the surface plasmon antenna, increasing propagation loss.